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The importance of warm-up time: BP Castrol Nexcel chart showing test results from simulated WLTP cycles using three European engines.

Optimizing engine oil warm-up strategies for 'real-world' driving

2016-09-09
Stuart Birch

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Optimizing engines for the new era of 'real-world' driving cycles will require new lubrication strategies—fast warm-up being a particular area of focus. A recent U.K. government survey showed that the average car journey time has fallen to only 22 minutes and average journey length to 12 km (7.5 m), most of it in urban areas. Such extreme duty cycles are increasingly typical in other global regions. They emphasize rapid warm-up for lowering emissions and fuel consumption, faster cabin heating and reduced maintenance.

The contribution to warm-up time due to heating the oil within the engine is often under-estimated, experts note.

“The specific thermal capacity of oil surprises many people because it’s substantially higher than for the metals used for the high-mass engine components," said Oliver Taylor, a chief engineer at BP Castrol. "Saving three liters (approximately 2.6 kg/5.7 lb) of oil is equivalent to shedding 6.4 kg (14 lb) from an aluminum block, or nearly 12 kg (26.5 lb) from an iron block. With that perspective, it becomes very clear how reducing the lubricant volume helps an engine to warm up more quickly.”

Less oil in the sump

High levels of internal friction during cold-start conditions are the dominant reason for increased fuel consumption and emissions during warm-up, which can comprise a significant proportion of the average journey. Taylor explained that in the new World Harmonized Light Vehicle Test Procedure (WLTP), up to 20% of the fuel energy is lost into warming up the metal parts, coolant and oil of a typical current-generation engine.

That engine’s oil volume throughout this period has to be sufficient to cater for the extremes required by a number of factors. In addition to that actually required for steady-state lubrication (i.e., the oil gallery requirement), the sump must contain sufficient lubricant to accommodate operation at a typical inclination of up to 30° from vertical to allow de-aeration when the engine is working at maximum speeds for prolonged periods and to achieve increasingly long oil-change intervals.

Another challenge comes from the increasingly complex lubricant additives required by today’s significantly downsized boosted engines. Additives "push up the viscosity, imposing a limit on viscosity reduction, however thin the base oil,” said Taylor. He told Automotive Engineering that electronic control of sump-oil volume can remove the need to always heat the full capacity required to accommodate the outer limits of these requirements.

BP Castrol testing shows that on a 2.0-L, highly-boosted, direct injection gasoline engine, more than two liters of oil can be removed from the engine lubrication circuit—effectively reducing the parasitic drag, or windage, that results from sump oil splashing on the cranktrain during operation. The reduction can significantly improve emissions and fuel consumption during most journeys.

Containing the oil within what Taylor terms an “intelligent cell” remote from the engine, also permits new approaches to the management of vital additives included within the oil formulation. One such approach is BP Castrol's self-contained, electronically managed sealed-cell system called Nexcel, is installed via a docking system (http://articles.sae.org/14426/) and can be swapped out within 90 s, according to Taylor, who led system development.

Governing the oil-additive content

The concept of a sealed-cell, easily changed engine oil module is increasingly viable within the complex issue of thermal management during warm-up, Taylor argues. He and colleagues presented results of an investigation into the effect of oil warm-up on CO2 emissions in a 2016 SAE Technical Paper (http://papers.sae.org/2016-01-0892/).

Nexcel has the capability to operate in a dry-sump architecture, but it can also be applied to a wet-sump installation, he explained. The system maintains sufficient oil in the engine’s oil pan to ensure adequate coverage of the oil pick-up, but retains the surplus within the cell. This "substantially reduces the thermal capacity of the oil circulating in the engine,” he said.

Contemporary engine oils contain up to 15% additives by weight, enabling them to remain effective for the extended oil-change intervals required by OEMs. But the additives' high viscosity contradicts the use of low-viscosity base stock to reduce engine friction. Taylor explained that the Nexcel unit "can be arranged to govern the additive system, allowing the engine to be supplied with oil containing 'tailored' additive content.

So far, BP Castrol is keeping secret details of the technique, but Taylor indicated that it is related to a new technology the company is developing based on "unique new chemistries” to actively control lubricant quality over the oil drain interval. “These techniques together will allow a precise and stable composition of the lubricant throughout the change interval,” claimed Taylor. He believes this is one of the major factors that will enable a significant further step towards highly optimized, vehicle-specific oils.

“Because the closed nature of the Nexcel system ensures the engine always receives the oil specified by engine designers, they can extend the envelope of possibilities in areas like bearing loads and temperatures while retaining robust durability margins," Taylor said. "Add the ability to manage oil quality through the change interval and you have a very powerful new tool for enabling new generations of downsized, highly-efficient engines.”

Oil-recycling benefit

The complexity and care needed to efficiently collect and manage the various oil grades drained from vehicles means that the vast majority of used oil becomes contaminated by other grades, or even by completely different fluids. This makes it impractical to re-refine, leading to a high proportion of recovered lubricants being used as fuel for burners that are “often of poor efficiency and questionable” environmental performance, noted BP Castrol Sustainability Director John Ward-Zinski.

“The benefit of controlling the feedstock for re-refining will be very significant," he explained, noting that just one-half liter of lubricant can be produced from 42 L of crude oil. BP’s research suggests 34 L of lubricant can be extracted from 42 L of recycled oil — "but only if you eliminate cross-contamination during the recycling process," Ward-Zinski said.

The sealed-cell system enables this "because it keeps individual oil types protected within their cells up to the point of re-refining.”